A technique for determining whether an outdoor unit is in trouble or an indoor unit is in trouble when an air conditioning system stops its operation due to a communication error is disclosed in Patent Document 1 and Patent Document 2. With the technique described in Patent Document 1, a determination device that includes a pair of terminals respectively connectable with an indoor unit and an outdoor unit, a photocoupler means connected between the terminals, and a display means connected on a transistor side of the photocoupler means is connected to the outdoor unit or the indoor unit to form a closed circuit, and it is determined whether an adequate current flows using a display means, thereby a failure in the outdoor unit or the indoor unit is detected. In other words, an external determination device is used after the operation of the air conditioning system has been stopped.
The technique described in Patent Document 2 discloses examples such that a return circuit that returns a signal as-is from an indoor unit is connected externally to the indoor unit and that the return circuit is provided within the indoor unit. It is determined that the outdoor unit is in failure if a determination signal that has been transmitted from the indoor unit and returned via the outdoor unit is not the same as an original determination signal, and it is determined that the indoor unit is in failure if a determination signal that has been transmitted from the indoor unit and returned as-is from the return circuit is not the same as the original determination signal.
It should also be noted that a technique for discriminating between a temporary failure such as a noise due to thunder and a failure over a communication path is disclosed in Patent Document 3.    Patent Document 1: Japanese Patent Application Laid-Open No. 09-149474    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-289495    Patent Document 3: Japanese Patent Application Laid-Open No. 2000-28187